1. Field of the Invention
The present invention relates to a hearing aid module for use in a mobile communication terminal particularly for the hard-of-hearing.
2. Description of Related Art
Mobile communication terminals equipped with hearing aid function for the hard-of-hearing have been increasingly demanded. According to FCC (Federal Communications Commission) regulation in the United States, mobile communication terminals equipped with no hearing aid function are not permitted to be sold since the year 2001 in the United States.
A hearing aid is a device used to help the hard-of-hearing hear sounds better. It includes a telecoil (T-coil) which is a coil of wire that is switched in place of a hearing aid microphone to allow the hearing aid to pickup the varying magnetic field at an earpiece of a telephone handset. The T-coil is essentially one half of a transformer, the other half being provided by the handset or an induction loop in a listening assistance system.
FIG. 1 is a schematic diagram of an ear speaker according to the prior art. An ear speaker 10 includes a permanent magnet 11 generating a magnetic field, a pole piece 12 forming a magnetic circuit in cooperation with the permanent magnet 11, a voice coil 13 wound around the permanent magnet 11 and the pole piece 12, a vibrating plate 14 provided over the pole piece 12, and a frame 15 and a cover 16 which cover the permanent magnet 11, the pole piece 12, the voice coil 13, and the vibrating plate 14. The ear speaker 10 is connected to an ear microphone of a mobile communication terminal (not shown) through a connecting terminal 17 to output an audio signal from the mobile communication terminal through the ear speaker 10.
In the ear speaker 10, the permanent magnet 11 generates a static magnetic field. When an audio signal is input from the mobile communication terminal, current flows through the voice coil 13. The current generates a dynamic magnetic field, thereby generating a Lorentz force. At this time, since the audio signal is an alternating current, its polarity is changed at very short intervals of approximately 1/1000 sec. The change in the polarity of the current causes the direction of Lorentz force to be changed, thereby creating forces of attraction and repulsion.
The magnitude of Lorentz force is proportional to the magnitude of flux density, the amount of current, and the length of the voice coil 13. The direction of Lorentz force is a direction perpendicular to a plane formed by the flux density and the current. Accordingly, the vibrating plate 14 moves up and down due to the Lorentz force, thereby producing a sound.
FIG. 2 is a schematic diagram of an ear speaker with hearing aid function according to the prior art. The ear speaker 10 includes a hearing aid coil (T-coil) 18 inside the cover 16. As shown in FIG. 2, a plurality of layers of the hearing aid coil 18 is provided over the voice coil 13 to produce a leakage magnetic field, thereby providing improved hearing performance.
In more detail, part of the magnetic field produced by the voice coil 13 leaks out of the ear speaker 10. The magnetic field produced by the voice coil 13 causes an induced electromotive force to be produced on the hearing aid coil 18. The electromotive force has the same frequency as but different magnitude from the audio signal inputted to the ear speaker 10.
Accordingly, since both an acoustic signal outputted from the voice coil 13 and an acoustic signal caused by the electromotive force induced on the hearing aid coil 18 are outputted from the ear speaker 10 with hearing aid function, the hard-of-hearing can hear sounds better through the ear speaker 10 with hearing aid function than through a typical ear speaker.
The ear speaker 10 with hearing aid function has a much larger impedance than a typical ear speaker with no hearing aid function to allow the hard-of-hearing to hear sounds better.
FIG. 9 is a frequency-response characteristic graph of an ear speaker with hearing aid function according to the prior art.
To increase the impedance of the ear speaker 10, the coil has to be thicker, the number of turns of the coil has to be increased, and the ear speaker itself has to be thicker to meet an optimum resonance condition.
However, there is a problem in that such a large-sized ear speaker is not convenient to use. Further, an ear microphone that allows a user to hear and input sounds is recently more popular than the ear speaker, so that it is more difficult to incorporate the hearing aid function into the ear microphone than the ear speaker.